1. Field of the Invention
The present invention relates generally to an Orthogonal Frequency Division Multiplexing (OFDM) system. In particular, the present invention relates to an apparatus and method for optimally compensating for imbalance between in-phase (I) and quadrature-phase (Q) in a zero-Intermediate Frequency (IF) OFDM receiver that downconverts a radio frequency (RF) signal directly to a baseband signal.
2. Description of the Related Art
OFDM is a prominent high-speed data transmission scheme for future-generation communications. Due to simultaneous transmission of mutually orthogonal subcarriers, OFDM has excellent frequency band efficiency. Also, the use of Cyclic Prefix (CP) effectively cancels Inter-Symbol Interference (ISI) caused by multipath fading, which is one of the most challenging problems in a radio communication environment.
Meanwhile, systems such as Digital Multimedia Broadcasting (DMB) or Digital Video Broadcasting-Handhelds (DVB-H) attempt a zero-IF approach to overcoming cost and system complexity constraints because IF Surface Acoustic Wave (SAW) filters make chip integration difficult. The use of the zero-IF technique is concentrated in the fields of mobile terminals or Wireless Local Area Networks (WLAN), which are presently highly competitive technologies.
While a combination of zero-IF reception architecture and OFDM enables the manufacture of receivers having excellent performance and a simple RF structure, I/Q imbalance inherent to a zero-IF receiver may cause performance degradation.
There are many traditional techniques for compensating for the I/Q imbalance in single-carrier systems. The most basic one is to design an analog circuit that creates variable gain and phase. Another is to generate a known signal in a receiving side, pass the known signal through an analog receiver, and estimate the distortion of the output, to thereby compensate for the I/Q imbalance.
Aside from those analog methods, there are I/Q imbalance correcting methods based on digital signal processing. A major one of them is to transmit a predetermined pilot signal from a transmitter and estimate the I/Q imbalance using the pilot signal at a receiver. A Minimum Mean Square Error (MMSE) I/Q imbalance compensation method, which is an expansion of the above method, uses an adaptive filter in correcting the I/Q imbalance instead of direct estimation of the I/Q imbalance using the pilot. This method advantageously achieves good performance, is able to track a change in the I/Q imbalance, and is implemented simply.
However, I/Q imbalance compensation using an additional I/Q imbalance estimation circuit at an analog receiver is likely to decrease the performance of a zero-IF receiver used with the aim to design an RF part as simple as possible for implementation of a reliable communication system. Moreover, I/Q imbalance compensation based on digital signal processing requires a pilot signal of a long period for accurate estimation, thereby increasing overhead. In view of the nature of analog devices, the I/Q imbalance value can be changed over time due to a variety of factors including temperature. Thus, for accurate I/Q imbalance estimation, the pilot signal needs to be transmitted periodically, which decreases transmission efficiency and imposes constraints on systems.
These problems can be solved by an MMSE I/Q imbalance compensation using a Least Mean Square (LMS) adaptive filter. This MMSE I/Q imbalance compensation technique requires an accurate reference signal for imbalance interference to obtain the tap coefficient of an adaptive filter. Since, in fact, only a signal including noise and affected by the I/Q imbalance can be generated at the receiver, it is impossible to optimally compensate for the I/Q imbalance using an MMSE compensator. This shortcoming does not matter to a low-level modulation such as Binary Phase Shift Keying (BPSK) or Quadrature Phase Shift Keying (QPSK), but serious performance degradation arises from the use of an inaccurate MMSE compensator in the case of multi-level Quadrature Amplitude Modulation (QAM).
OFDM is considered for extensive use as a future-generation communication scheme in various fields, and adopts schemes for adaptively using multi-level QAM to increase band efficiency. In the situation where the zero-IF receiver attracts more and more interest due to its benefits in terms of chip integration, the I/Q imbalance problem of a zero-IF OFDM receiver significantly decreases the performance of QAM modulation/demodulation. Although the I/Q imbalance can be compensated for by use of an MMSE compensator, optimum performance is impossible with the tap coefficient of the MMSE compensator derived from an inaccurate reference signal.